Vehicle engine systems may include a turbocharger that uses the exhaust flow to increase the boost pressure of the intake air that is supplied to the engine. Specifically, a turbocharger may have a compressor section with a compressor wheel, and a turbine section with a turbine wheel and a diffuser downstream of the turbine wheel. The flow of exhaust gas through the turbine section may cause the turbine wheel to rotate and drive the rotation of the compressor wheel through an interconnecting shaft. The rotating compressor wheel may pressurize the intake air that is supplied to the engine through the intake manifold.
At the design point of the turbine section, the flow of the exhaust gas exits the turbine wheel with little to no swirl/rotation relative to the turbine housing. The design point represents the mass flow that yields the peak operation efficiency of the turbine section at a given speed. As the mass flow deviates from the design point, the turbine section is considered to be operating under off-design conditions in which the flow of the exhaust gas exiting the turbine wheel exhibits rotation relative to the turbine housing. The swirling or rotating exhaust gas flow exiting the turbine wheel under off-design conditions may move radially outward toward the limits of the diffuser due to the conservation of angular momentum.
Many diesel engine systems include an aftertreatment system in the exhaust pipe to remove or reduce the level of certain pollutants in the exhaust gas stream. Such aftertreatment systems use various catalysts that selectively convert targeted pollutants in the exhaust gas stream. Engine manufacturers are increasingly positioning aftertreatment catalysts, such as diesel oxidation catalysts (DOCs), closer to the discharge of the turbine section. For instance, a DOC catalyst may be placed immediately downstream of the downstream end of the turbine diffuser, or may even protrude into the turbine diffuser. This arrangement advantageously provides the catalyst with high temperature exhaust gases as the gases exit the turbine wheel, promoting catalyst light off and catalytic conversion of targeted pollutants in the exhaust gas stream. However, under off-design conditions, the swirling/rotating exhaust gases may move radially outward toward the outer edges of the catalyst. As such, the exhaust gas may not be uniformly distributed across the face of the catalyst under off-design conditions, leading to reduced catalytic efficiency.
U.S. Patent Application Number 2016/0245119 discloses a turbocharger diffuser having a center body within the diffuser supported by deswirl vanes extending from a wall of the diffuser. The center body and the deswirl vanes create a deswirl passageway for the exhaust gases flowing through the diffuser. While effective, the reference does not mention the use of the deswirl passageway to promote an even distribution of exhaust gas flow across the face of a downstream aftertreatment catalyst.
Thus, there is a need for strategies for improving the uniformity of exhaust gas flow to aftertreatment catalysts that are positioned near the turbine section discharge.